B cell antigen receptor (BCR) signaling is not only critical for antibody-mediated immunity to pathogens but also maintains tolerance to self. Hyperactive BCR signaling is a hallmark of autoimmune diseases such as systemic lupus erythematosus (SLE), but the molecular mechanisms controlling B cell hyper responsiveness to antigen are poorly understood. Therefore, identification of proteins that drive B cell hyperactivation and autoantibody production is crucial for development of effective therapeutic strategies in SLE. Lyn-/- mice display B cell hyperactivation as well as pathogenic symptoms associated with human SLE. Human GWAS studies have shown a genetic association between the Src family kinase Lyn and SLE, making Lyn-/- mice a clinically relevant model for this disease. We have previously reported that the membrane-cytoskeleton linker protein ezrin regulates membrane dynamics and BCR signaling in antigen-stimulated B cells through its ability to undergo dynamic changes in phosphorylation. Interestingly, conditional deletion of ezrin in B cells of Lyn-/- mice leads to decreased B cell activation and differentiation, and reduced autoantibody production. These data suggest that ezrin-mediated regulation is a critical mechanism governing the hyper activation of B cells in the absence of Lyn. Ezrin is hyperphosphorylated in Lyn-/- B cells, and dephosphorylation of ezrin leads to reduced BCR microclustering, calcium signaling and cytokine secretion. Our data suggest a positive feedback model whereby stronger calcium signaling in Lyn-/- B cells leads to increased calcium-dependent protein kinase C activation, which results in hyperphosphorylation of ezrin, which in turn increases BCR microclustering and signaling. Collectively, our data reveal a novel collaborative interface between ezrin and Lyn that regulates the strength of BCR signaling in SLE, and may be a potential target for therapeutic intervention. We hypothesize that ezrin regulates B cell hyper activation and autoimmunity in Lyn-/- mice via its ability to regulate BCR spatial organization and signaling. Our specific aims are, (1) To determine the impact of ezrin deletion on development of B cell autoimmunity in Lyn-/- mice, (2) To investigate the effect of ezrin deletion on BCR organization and signaling in Lyn-/- B cells, and (3) To elucidate the mechanism by which hyperphosphorylated ezrin mediates hyperactivity of Lyn-/- B cells. We will employ genetic, biochemical, molecular biological, immunological and cutting-edge high resolution imaging techniques to accomplish our aims. Our results will provide insights into spatial regulation of BCR response to antigen in lupus B cells, and elucidate the mechanism by which ezrin mediates the hyperactivity of Lyn-/- B cells. Ultimately, our findings will lead to the development of novel strategies for treatment of lupus and other B cell disorders that exhibit aberrant BCR signaling.